An active matrix substrate is widely used in an active matrix type display device such as a liquid crystal display device and an EL (Electroluminescence) display device. For example, the main part of the active matrix type liquid crystal display device is composed of a liquid crystal panel and drive circuits thereof, and the liquid crystal panel typically includes: the active matrix substrate having pixel circuits disposed in a matrix, each of which contains a thin film transistors (hereinafter, simply abbreviated as “TFT”) as a switching element and a pixel electrode and the like; an opposite substrate of a transparent insulating substrate such as a glass plate having an opposite electrode and an alignment film sequentially deposited over the entire surface thereof; a liquid crystal layer held between the both substrates; and polarizers attached to the respective outside surfaces of the both substrates.
FIG. 29 is a plan view showing a structure of a conventional active matrix substrate 700 used for the liquid crystal display device as described above, and illustrates a pattern configuration in a part corresponding to one pixel. The active matrix substrate 700 includes a plurality of data signal lines 715, a plurality of scanning signal lines 716 intersecting the plurality of data signal lines 715, TFTs 712 formed as the switching elements in the neighborhood of the intersections of the plurality of data signal lines 715 and the plurality of scanning signal lines 716, respectively, and a pixel electrode 717. The scanning signal line 716 works also as the gate electrode of the TFT 712, the source electrode 719 of the TFT 712 is connected to the data signal line 715, and the drain electrode 708 is connected to the pixel electrode 717 via a drain extraction electrode 707. A hole is formed in an insulating film disposed between the drain extraction electrode 707 and the pixel electrode 717 and thereby a contact hole 710 is formed connecting the drain extraction electrode 707 and the pixel electrode 717. The pixel electrode 717 is a transparent electrode made of ITO (Indium Tin Oxide) or the like and transmits light from the back of a liquid crystal panel including the active matrix substrate 700 (backlight).
In the active matrix substrate 700, the TFT 712 is turned on by a gate-on voltage of a scanning signal applied to the scanning signal line 716 (conductive state between the source electrode 719 and the drain electrode 708), a data signal applied to the data signal line 715 in this state is written into a pixel capacitance (capacitance formed by the pixel electrode 717 and the opposite electrode) via the source electrode 719, the drain electrode 708, and the drain extraction electrode 707. Note that, in the active matrix substrate 700, a storage capacitance line 718 is formed along the scanning signal line 716 and the storage capacitance line 718 has a function of avoiding self discharge of the liquid crystal layer during an OFF period in the TFT 712, or the like.
The liquid crystal display device using such an active matrix substrate 700 is typically driven by an alternating voltage for preventing the deterioration of display quality, and a technique is proposed (e.g., Japanese Patent Application Laid-Open Publication No. 8-43795 (Patent document 2)), in which a positive polarity voltage and a negative polarity voltage are applied to the pixel electrode alternately at a fixed time interval with an opposite voltage as a reference potential, the opposite voltage being applied on the opposite electrode (also called “common electrode”) provided on the opposite substrate opposite to the active matrix substrate 700 sandwiching the liquid crystal layer, and the polarity inverts every two horizontal periods, for example (hereinafter, called “2H inversion drive”).
However, while in the drive of the first line of the two lines which are a unit for the polarity inversion in this 2H inversion drive, the polarity of the applied voltage to the data signal line is inverted immediately before the drive of the first line is started, the polarity of the applied voltage to the data signal line is not inverted when the drive is shifted from the first line to the second line in the two lines. Therefore, the drive of the first line requires a longer time for charging the data signal line compared to the drive of the second line, and resultantly a difference of charged amount in the pixel capacitance is caused between the first line and the second line. This difference of the charged amount appears as a luminance difference between a pixel in the Nth line corresponding to the first line of the polarity inversion unit and a pixel in the (N+1)_th line corresponding to the second line of the polarity inversion unit in one frame and causes horizontal line irregularity to be viewed.
Accordingly, there is proposed a method to make uniform the charging characteristic by applying the data signal exhibiting a certain intermediate potential between the positive polarity and negative polarity potentials during a blanking period for every horizontal period (Japanese Patent. Application Laid-Open Publication No. 2004-61590 (Patent document 3))    Patent document 1: Japanese Patent Application Laid-Open Publication No. 9-152625    Patent document 2: Japanese Patent Application Laid-Open Publication No. 8-43795    Patent document 3: Japanese Patent Application Laid-Open Publication No. 2004-61590    Patent document 4: Japanese Patent Application Laid-Open Publication No. 9-243998    Patent document 5: Japanese Patent Application Laid-Open Publication No. 2002-268613